The present invention relates to processes and apparatus for purification of brine generated by industrial processes. Purified brine may be used in industrial processes such as the chlor-alkali process for electrolytic conversion of brine to chlorine gas or hypochlorite and sodium hydroxide.
Brine is generated by industrial processes that react chlorine atom-containing compounds with an inorganic base such as sodium hydroxide to form an aqueous brine solution containing chloride salts. Examples include the production of epichlorohydrin by reacting chlorohydrins with sodium hydroxide, the production of epoxy resins by reacting epichlorohydrin with polyphenolic compounds, such as bisphenol A or bisphenol F, in which the base reacts with chlorine atoms of the epichlorohydrin and the phenolic hydrogen atoms, and scrubbing of industrial effluent to remove hydrogen chloride from a chemical stream by reacting the hydrogen chloride with sodium hydroxide, such as in the hydrogen chloride absorber used to remove hydrogen chloride during the phosgenation process used to make isocyanates. The aqueous brine solutions produced by such processes often contain one or more organic compounds associated with the process(es) from which the brine is derived.
Aqueous brine solutions containing sodium chloride as the predominant salt are useful for the production of chlorine gas or hypochlorite and sodium hydroxide by an electrolytic process known as the chlor-alkali process. Chlorine gas, hypochlorite and sodium hydroxide produced by the chlor-alkali process are useful in a number of industrial processes in which chlorine atoms and/or a strong base is required. It would be desirable to be able to use aqueous brine solutions produced by industrial processes in the chlor-alkali process to integrate industrial chemical processes and thereby reduce raw material acquisition and byproduct disposal costs.
A problem associated with using aqueous brine solutions produced by industrial processes in the chlor-alkali process is that the presence of impurities such as organic compounds in such aqueous brine solutions must generally be reduced to a very low concentration, because the chlor-alkali process has a low tolerance for impurities, including organic compounds. Generally, the organic compound concentration in aqueous brine used in industrial chlor-alkali production should be less than 50 ppm, and preferably should be less than 10 ppm, total organic carbon (TOC).
A known method for reducing the organic compound concentration in aqueous brine solutions is to conduct ozonolysis to oxidize organic compounds to more volatile oxidation fragments and/or carbon dioxide that can be stripped from the aqueous brine solution. Ozonolysis is generally carried out by introducing ozone gas or ozone dissolved in water into the aqueous brine solution at an elevated temperature. Such a process is disclosed, for example, in U.S. Pat. No. 6,340,736.
Another known method for reducing the organic compound concentration in aqueous brine solutions is to conduct chlorinolysis to oxidize organic compounds to more volatile oxidation fragments and/or carbon dioxide that can be stripped from the aqueous brine solution. Chlorinolysis is generally carried out by introducing chlorine gas or hypochlorite into the aqueous brine solution at an elevated temperature. Such a process is disclosed, for example, in U.S. Pat. No. 4,240,885.
A disadvantage of relying solely on chlorinolysis for removal of organic compounds is that substantial amounts of chlorine gas or hypochlorite is generally required to reduce the organic compound concentration to an acceptable level when the initial organic compound concentration prior to chlorinolysis is relatively high. In that case, the purification process consumes a substantial portion of the chlorine gas or hypochlorite generated by the chlor-alkali process to thereby reduce the availability of the chlorine gas or hypochlorite generated by the chlor-alkali process for other industrial processes.
Another disadvantage of relying solely on chlorinolysis is that compounds containing oxygen atoms such as alcohols, acids, esters, aldehydes, ketones, and ethers are generally more difficult to oxidize to break them down into oxidation fragments sufficiently volatile to be stripped from the aqueous brine solution. Reducing the concentration of such oxygen-containing compounds to an acceptable level via chlorinolysis is difficult and expensive.
Another disadvantage of relying solely on chlorinolysis is that it requires treatment of the vapor stream stripped from the brine solution to prevent discharge of chorine gas, hypochlorite and any chlorinated hydrocarbons into the environment.
Opportunities therefore remain to further improve the purification of aqueous brine solutions containing organic compounds so that the brine can be used for chlor-alkali electrolysis.